In a skillet, brown the ground beef over medium heat until fully cooked.
Add the chopped onion and bell pepper, sautéing until soft.
Drain excess fat from the skillet.
Stir in the ketchup, Worcestershire sauce, brown sugar, and mustard.
Season with salt and pepper to taste.
Simmer the mixture for 10 minutes, stirring occasionally.
While simmering, toast the hamburger buns until golden.
Once the mixture is thickened, remove from heat.
Place a generous scoop of the beef mixture onto each bun.
Garnish with fresh parsley and serve immediately.